NATURAL SELECTION 247 



that a perfect and complex eye could be formed by natural selection, 

 though insuperable by our imagination, should not be considered as 

 subversive of the theory. How a nerve comes to be sensitive to light, 

 hardly concerns us more than how life itself originated; but I may 

 remark that, as some of the lowest organisms, in which nerves cannot 

 be detected, are capable of perceiving light, it does not seem impossible 

 that certain sensitive elements in their sarcode should become aggre- 

 gated and developed into nerves, endowed with this special sensi- 

 bility. 



In searching for the gradations through which an organ in any 

 species has been perfected, we ought to look exclusively to its lineal 

 progenitors; but this is scarcely ever possible, and w are forced to 

 look to other species and genera of the same group, that is to the 

 collateral descendants from the same parent-form, in order to see what 

 gradations are possible, and for the chance of some gradations hav- 

 ing been transmitted in an unaltered or little altered condition. But 

 the state of the same organ in distinct classes may incidentally throw 

 light on the steps by which it has been perfected. 



The simplest organ which can be called an eye consists of an optic 

 nerve, surrounded by pigment-cells and covered by translucent skin 

 but without any lens or other refractive body We m.ay, however, 

 according to M. Jourdain, descend even a step lower and find aggre- 

 gates, of pigment-cells, apparently serving as organs of vision, without 

 any nerves, and resting merely on sarcodic tissue. Eyes of the above 

 simple nature are not capable of distinct vision, and serve only to dis- 

 tinguish light from darkness. In certain star-fishes, small depressions 

 in the layer of pigment which surrounds the nerve are filled, as de- 

 scribed by the author just quoted, with transparent gelatinous matter, 

 projecting with a convex surface, like the cornea in the higher animals. 

 He suggests that this serves not to form an image, but only to con- 

 centrate the luminous rays and render their perception more easy. 

 In this concentration of the rays we gain the first and by far the most 

 important step towards the formation of a true, picture-forming eye^ 

 for we have only to place the naked extremity of the optic nerve, 

 which in some of the lower animals lies deeply buried in the body, and 

 in some near the surface, at the right distance from the concentrating 

 apparatus, and an image will be formed on it. 



In the great class of the Articulata, we may start from an optic 

 nerve simply coated with pigment, the latter sometimes forming a sort 



